Estimation of σ-Donation and π-Backdonation of Cyclic Alkyl(amino

Article pubs.acs.org/IC

Estimation of σ‑Donation and π‑Backdonation of Cyclic Alkyl(amino) Carbene-Containing Compounds Kartik Chandra Mondal,*,† Sudipta Roy,† Bholanath Maity,‡ Debasis Koley,*,‡ and Herbert W. Roesky*,† †

Institut für Anorganische Chemie, Georg-August-Universität, Tammannstraße 4, 37077 Göttingen, Germany Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

‡

S Supporting Information *

ABSTRACT: Herein, we present a general method for a reliable estimation of the extent of π-backdonation (CcAAC←E) of the bonded element (E) to the carbene carbon atom and CcAAC→E σ-donation. The CcAAC←E π-backdonation has a significant effect on the electronic environments of the 15N nucleus. The estimation of the π-backdonation has been achieved by recording the chemical shift values of the 15N nuclei via two-dimensional heteronuclear multiple-bond correlation spectroscopy. The chemical shift values of the 15 N nuclei of several cAAC-containing compounds and/or complexes were recorded. The 15N nuclear magnetic resonance chemical shift values are in the range from −130 to −315 ppm. When the cAAC forms a coordinate σ-bond (CcAAC→E), the chemical shift values of the 15N nuclei are around −160 ppm. In case the cAAC is bound to a cationic species, the numerical chemical shift value of the 15N nucleus is downfield-shifted (−130 to −148 ppm). The numerical values of the 15N nuclei fall in the range from −170 to −200 ppm when σ-donation (CcAAC→E) of cAAC is stronger than CcAAC←E π-backacceptance. The π-backacceptance of cAAC is stronger than σ-donation, when the chemical shift values of the 15N nuclei are observed below −220 ppm. Electron density and charge transfer between CcAAC and E are quantified using natural bonding orbital analysis and charge decomposition analysis techniques. The experimental results have been correlated with the theoretical calculations. They are in good agreement.

O

ver the past two and a half decades, stable singlet carbenes have been utilized as efficient ligands in different fields of chemistry.1 Stable carbenes possess a divalent carbon atom that is bonded to at least one heteroatom (and to a maximum of two heteroatoms); otherwise, they cannot be isolated or stored under ambient conditions because of the electron deficiency at the carbene carbon center.2 The heteroatom (such as N, P, S, or O) reduces the electron deficiency of the carbene carbon atom by sharing its lone pair of electrons.2a NHCs (NHC = N-heterocyclic carbene) generally stabilize chemical species via strong σ-donation from the carbene carbon atoms to the bonded elements (CNHC→E).3 The role of carbenes as ligands for the stabilization of low-valent main group elements and transition metals in their low oxidation states has been demonstrated.3 Moreover, carbenes have been shown to function as efficient ligands for transition metals in the field of catalysis.1,4 The syntheses and characterizations of carbene-stabilized chemical species have an important influence on the rapidly growing areas of chemistry.5 Initially, N-heterocyclic carbenes (NHCs) have been employed as strong σ-donating ligands.1−3 Today, cyclic alkyl(amino) carbenes (cAACs) have been introduced as competing ligands (Figure 1).6 In cAAC, the HOMO is slightly higher and the LUMO is slightly lower in energy when compared with those of NHC.7 Consequently, the HOMO− LUMO energy gap is smaller in cAAC than in NHC, and this is © 2015 American Chemical Society

Figure 1. Comparison between NHC (left) and cAAC (right) carbenes.

the most crucial aspect of cAAC. A couple of years ago, the comparative σ-donation and π-acceptance properties of carbenes were studied in a series of carbene−phosphenidene adducts.8 cAACs have been shown to be stronger σ-donors and better π-acceptors than NHCs,9 making them more suitable5−8 for the stabilization of chemical species that have not been isolated by employing NHCs as ligands. Recently, it was Received: September 9, 2015 Published: December 16, 2015 163

DOI: 10.1021/acs.inorgchem.5b02055 Inorg. Chem. 2016, 55, 163−169

Article

Inorganic Chemistry demonstrated that cAAC ligands have stabilized several monatomic first row transition metals (Zn−Mn) with coordination number two.10 The cAAC-ligated diatomic molecules (Au2 and Co2)11a,b and the anionic Fe− species11c have been reported. These systems (Au2 and Co2)11a,b are unique for the study of their metal−metal interactions. The strong π-accepting properties of cAACs induce diradicaloid character in silylones,12a germylones,12b and the carbon monoxide adduct.12c Over half of a decade, a large number of radicals and diradicals containing cAACs were isolated and studied by electron paramagnetic resonance (EPR) spectroscopy.13 The paramagnetic radicals are generally characterized by EPR measurements rather than by using nuclear magnetic resonance (NMR) spectroscopy. However, NMR spectroscopy is utilized as one of the very important tools for the characterization of diamagnetic cAAC-containing species. The chemical shift values of 1H, 13C, and 31P NMR are among the most commonly recorded data for the characterization of these compounds. The most utilized carbenes contain nitrogen as heteroatoms. However, the 15N (I = 1/2) or 14N (I = 1) NMR chemical shift values have seldom been recorded for the characterization of carbene-containing compounds. The 15N NMR chemical shift values of free NHC and some NHC-containing complexes have been reported elsewhere.14 Surprisingly, these values were not correlated with the π-backaccepting property of NHC ligands. Experimental results suggest that the carbene carbon atom of cAAC polarizes the electron densities from the adjacent nitrogen atom to its vacant pz orbital (N→CcAAC). This electron sharing depends on the electron density on the element (E) bound to the cAAC and their oxidation states.10−13 Geometrically, this is reflected in the C−N bond distance that varies in the range of 1.28−1.46 Å. The cAACs form stronger donor−acceptor bonds (CcAAC→E and CcAAC←E), which are responsible for the better stability of the resulting products.15,16 Structural bond parameters and theoretical investigations (NBO analysis) of cAAC-containing products often suggest that the π-backdonation from the bound element (E) to the carbene carbon atom (CcAAC←E) is stronger than the σ-donation from cAAC to the element E (CcAAC→E), which leads to a positive partial charge on E (E = main group element or metal).10,11,12a,b The cAAC favors a CcAAC→E bond when E is higher-valent,13a while it prefers a combination of CcAAC→E and E→CcAAC when E is lowervalent.15,16 Switching of the type of CcAAC−E bond is achieved by oxidation or reduction of the E/ELn part (L = halogen atoms).13,15−17 Another type of C−E bond is observed in cAAC-containing radicals, in which the CcAAC forms a covalent electron sharing single bond13 with E along with one unshared electron (radical) on the CcAAC. There is no such precedence of this type of bonding in case of NHC. The cAAC has the unique ability to control the distribution of electron densities around the bound element. Such a feature was not observed in NHC. The significant π-accepting property makes cAAC in many ways different from NHC.18 Herein, we report on the theoretical and experimental 15N NMR spectroscopic studies (HMBC, three-bond coupling of 1H−15N) to estimate the amount of σ-donation and π-backacceptance of cAAC in some of the representative cAAC-containing compounds. cAAC has a singlet-spin ground state with a pair of electrons on the sp2 orbital of the carbene carbon atom (CcAAC) (Figure 2, left) and a vacant p-orbital on CcAAC. The CcAAC−N bond

Figure 2. Three-bond 1H−15N coupling (left) and σ-donation of cAAC and π-backdonation from E (right).

distance of a free cAAC is shorter (1.315 Å) than the normal C−N single bond due to the delocalization of the lone pair of electrons of the adjacent nitrogen atom to the pz orbital of the carbene carbon atom (N:→CcAAC). As mentioned earlier, experimentally it has been observed that compounds containing cAAC as a ligand can have a wide range of bonding scenarios, e.g., coordinate σ-bond, donor−acceptor partial double bond, and covalent electron-sharing single bond. The CcAAC−N bond distances are recorded within the range of 1.28−1.46 Å. The CcAAC−N and CcAAC−E bond distances often infer to the nature of the bond existing between CcAAC and the acceptor atoms. However, there are exceptions. It has been reported in some cases that the CcAAC−E bond distances are similar, although the natures of bonds between CcAAC and E are completely different. For example, the monoradical (cAAC•)SiCl3 has a bond distance (CcAAC−Si) similar to that of (cAAC)2Si2Cl2 (∼1.82 Å). However, the former contains an electron-sharing covalent CcAAC−Si single bond between silicon and the carbene carbon atom, while the latter has a donor−acceptor type partial double (CcAACSi) bond. The CcAAC−N bond distances of (cAAC•)SiCl3 and (cAAC)2Si2Cl2 are slightly different. The difficulty in differentiating the nature of the CcAAC−E bonds mentioned above has rarely16 been experimentally addressed. Although, Bertrand et al. have experimentally observed a general trend of the π-accepting property of different carbenes in a series of carbene−phosphenidene adducts, a correlation between the nature of the chemical bond and an easily measurable observable is in general lacking.8 It is obvious that a change in the nature of the bonding should have an effect on the 14N or 15N nucleus. The former nucleus is more sensitive than the 15N one. The NMR resonances are usually broadened by quadrupolar interactions, making the characterization via a high-resolution NMR spectrometer difficult. Therefore, two-dimensional heteronuclear multiple-bond correlation spectroscopy (1H−15N HMBC) has been employed to extract more information about the nature of the bond between CcAAC and E (Figure 2, right). Using this method, chemical shift values of free cAAC and cAAC-containing compounds of main group elements (mostly silicon, E = Si) have been recorded. The 15N HMBC (three-bond correlation) measurement has been started with cAAC·LiOTf (1),6 which shows a resonance at −159.0 ppm coupling with protons of the CH2 unit of the five-membered carbene ring and methyl protons of the NCMe2 group (as shown in Figure 2). The cAAC reacts with SiCl4 to produce the (cAAC)→SiCl4 (4)13a adduct that shows a 15N resonance at −164.1 ppm, suggesting the carbene carbon atom of cAAC forms a coordinate σ-bond with both LiOTf and SiCl4 in 1 and 4, respectively (Figure 3). The coordinate bond in 1 is very strong, and hence, it is very difficult to extract the free cAAC (Me2-cAAC, Et2-cAAC, and Cy-cAAC) by using 164

DOI: 10.1021/acs.inorgchem.5b02055 Inorg. Chem. 2016, 55, 163−169

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Inorganic Chemistry

Figure 3. Representative cAAC-containing compounds with chemical shift values (δ) and their bonding characteristics.

partial double bonds.15,16 The 15N NMR resonances are observed at −208.5 and −206.0 ppm for 6 and 7, respectively, which are upfield-shifted when compared with that of precursor 4, suggesting both CcAAC→Si σ-donation and CcAAC←Si πbackdonation in 6 and 7 (Figure 3). Replacement of Me2-cAAC with Et2-cAAC or Cy-cAAC has a negligible effect on the 15N chemical shift values. The 15N chemical shift values of cAAC-containing cationic compounds are found to be downfield-shifted compared to those of cAAC•(LiOTf) (1). The CcAAC←P π-backdonation becomes negligible in the cationic salt [(cAAC)PPh2]+Cl− (8).13e The cation PPh2+ is stabilized by the CcAAC→P σdonation, and the lone pair of electrons is tightly retained on the phosphorus atom, leading to a shorter C−N bond and a longer C−P bond. The 15N resonance of 8 appears at −130.3 ppm, which is downfield-shifted with respect to that of 1 because of the cationic charge on the carbene carbon atom of (cAAC)PPh2+. The corresponding 15N chemical shift value of the protonated carbene salt cAAC•H+ OTf− (3) is −148.1 ppm, which is upfield-shifted when compared with that of 8. This might be due to the lower electronegativity of the hydrogen atom. The 15N resonances of (cAAC)O (9),19 (cAAC)S (23), and cAAC→Se (19) are observed at −241.0, −199.8, and −187.5 ppm, respectively, suggesting a CO/ CS double bond character in 9 and 23, while the corresponding carbon−selenium bond is likely to be a CcAAC→Se σ-bond in 19. The accumulation of electronic charge on the selenium atom of cAAC→Se is higher, and therefore, it is easily utilized by selenium to ionize iodine (I2) to form the cAAC→Se−I−I+ I3− salt.20 When the 1,3-diradical (cAAC•)2SiCl2 (10) is completely dechlorinated with 2 equiv of KC8, siladicarbene (cAAC)2Si (11) is obtained.12a Theoretical calculations showed that the covalent C−Si single bond in 10 has transformed to a C−Si−C three-center, two-electron π-bond in 11. The accumulation of positive charge on the central silicon atom of 11 suggests that the π-backdonation from silicon to the carbene carbon atom (CcAAC←Si) is greater than the CcAAC→Si σ-donation.12a The 15 N chemical shift value of 11 is −230.5 ppm, which is more upfield-shifted compared with that of disiladicarbene (7)16 but close to that of the analogous (cAAC)2Ge (germylone)12b (−222.5 ppm). The electron-induced isomerization of 11 leads to the formation of compound (Cy-cAACH)Si(CMe2)(CycAAC) (12),13d which is theoretically predicted to have a higher degree of donor−acceptor carbon−silicon double bond character because of the better CcAAC←Si π-backdonation. The experimental chemical shift of 12 is further upfield-shifted (−256.5 ppm), as expected (Figure 3). It is well-known that silicon is a metalloid, and hence, the silicon−carbon bond is expected to have covalent character greater than that of a metal−carbon bond.10c,12a,d The 15N chemical shift values of some of the carbene−metal complexes (Figure 4) have been recorded to study the effect of σ-donation (CcAAC→M) of cAAC and π-backdonation (CcAAC←M) of the metal (M) and confirm the validity of the general trend. The bond between CcAAC and nickel in the square planar complex (cAAC)2NiCl2 (13)10c is a CcAAC→Ni coordinate σ-bond [C− N, 1.3154(18) Å; C−Ni, 1.9150(14) Å]. The 15N chemical shift (−160.6, −161.0 ppm; two conformers) of 13 is close to the value of 1. The theoretical calculations on (cAAC)2Ni (14) [C−N, 1.3381(16)/1.3420(16) Å; C−Ni, 1.8448(14)/ 1.8419(13)]10c showed the presence of significant π-backdonation (CcAAC←Ni0). The electronegativities of Si and Ni are

15

N

nonpolar solvents such as n-hexane or toluene. Only bulky cAAC ligands can be extracted (free from LiOTf) with nonpolar solvents.6 When cAAC forms a coordinate bond (CcAAC→E), the CcAAC−N bond distance is close to 1.30 Å. Compound (cAAC)→Si(Cl2)→P-Tip (2)18c shows a resonance at −160.7 ppm that is close to those of 1 and 4 (Figure 3), as expected. As a matter of fact, the carbon−silicon (CcAAC→Si) coordinate σ-bond distance is approximately ∼1.94 Å, in 2 and 4. The monoradical (cAAC•)SiCl3 is produced when (cAAC)→SiCl4 (4) undergoes one-electron reduction.13a The bond between the carbene carbon atom and silicon atom results in a covalent electron-sharing single bond (CcAAC−Si) leading to a shortening of the carbon−silicon bond distance by ∼0.12 Å and lengthening of the CcAAC−N bond by ∼0.09 Å.13a A 1,4-singlet diradical (Cy-cAAC•) (Cl2)Si−Si(Cl2)(•CAAcCy) (5) is formed when (cAAC•)SiCl3 is further reduced with 1 equiv of KC8.13c Singlet 1,3-diradical (cAAC•)2SiCl2 (10) is obtained via the metathesis of (IPNHC)→SiCl2 with 3 equiv of cAAC.13b However, both monoradical (cAAC•)SiCl3 and diradicals 5 and 10 are 15N NMR silent because of the presence of the radical electron on the carbene carbon atoms. Compounds with low-coordinate silicon, (cAAC)→Si(Cl)− Si(Cl)←(cAAC) (6)15 and (Me2-cAAC)→SiSi←(CAAcMe2) (7),16 are synthesized via the reduction of (cAAC)→ SiCl4 (4) with 3 and 4 equiv of KC8, respectively. The C−N bond distances are similar in both 6 and 7, while the CcAAC−Si bond distance is slightly shorter in 6. Both 6 and 7 are theoretically suggested to possess donor−acceptor coordinate 165

DOI: 10.1021/acs.inorgchem.5b02055 Inorg. Chem. 2016, 55, 163−169

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Inorganic Chemistry

Figure 4. Representative diamagnetic cAAC compounds with chemical shift values (δ).

to that of free iPrNHC. These values are even slightly downfield-shifted when compared with that of iPrNHC·HCl. The chemical shift value of (cAAC)2NiCl2 (13) [−160.6/− 161.0 ppm (Figure 4)] is far more downfield-shifted compared to that of the NHC analogue.10c,23 (iPrNHC)2Ni is obtained when (iPrNHC)2NiCl2 is reduced with 2 equiv of KC8 in a mixture (5:1) of solvents (toluene/THF). The 15N NMR of (iPrNHC)2Ni is observed at −197.5 ppm, which is close to the reported value of (MesNHC)2Ni (−193.9 ppm).14f The chemical shift values of NHC complexes given above are in line with the previously made24 statement that NHC is a poor π-acceptor. The DFT calculations have been performed (for details, see Computational Details in the Supporting Information) on complexes 1−17, 19, 20, and 22 to calculate theoretical 15N chemical shift (δ) values, correlated with structural parameters. The calculated δ values at the R/U-B3LYP/def2-TZVP level of theory are very similar to the experimental findings (Table 1). The natural bond orbital (NBO) analysis that is implemented in Gaussian09 has been studied to gain insight into the electronic structure and bonding nature of CcAAC−E and N− CcAAC bonds. In most of the compounds, the CcAAC is preferably connected to the E by σ-donation with concomitant π-acceptance to its vacant p-orbital to increase the stability.15,16 The CcAAC can also share the lone pair of electrons from the adjacent N atom to reduce its electron deficiency. Therefore, the electronic environment around the N atom is mainly regulated by the electron deficiency on the CcAAC, which further depends upon the π-backdonation from the E atom. To quantify the extent of CcAAC→E σ-donation, CcAAC←E πbackdonation, and N→CcAAC π-donation, we performed natural localized molecular orbital (NLMO) analysis (Table 1). Unsurprisingly, the electron density of the CcAAC−E σ-bond mostly resides on the carbene atom and the reverse is true for the CcAAC−E π-bond. The delocalization of the lone pair electrons of the N atom toward CcAAC (N→CcAAC) leads to another π-acceptance situation. Therefore, the extents of these two π-bonds (CcAAC←E and N→CcAAC) are inversely related to each other. The natural bond order and Wiberg bond order of CcAAC−E and N−CcAAC bonds were calculated to evaluate the π-bond strength (Table S1). According to NBO results, there is no significant CcAAC←E π-back-donation observed in compounds 1−5, 8, and 10 and the hydrogenated (cAACH) carbene part of 12. In molecules 9, both the σ- and π-bonding electrons are polarized toward the O atom because of the higher electronegativity indicating a higher degree of covalency. A similar type of covalency occurs in the CSe bond of 19 and the CSi part of 12, which is obvious from the NLMO data (Table 1). Moreover, from Table 1, it is clear that in the CcAAC−E donor−acceptor bond, CcAAC shows greater σdonation than π-acceptance, while an opposite result was found in the case of compound 11 where π-backdonation (31%) is predominant over σ-donation (23%), which is further supported by a previous report.12a The positive partial charges on the Si atoms in 7 (qSi = 0.109e) and 11 (qSi = 0.546e) also indicate that the CcAAC←Si π-backdonation is stronger than the CcAAC→Si σ-donation (Table 1 and Table S2). However, it is evident that the percentage of N→CcAAC π-donation value is controlled by CcAAC←E π-backdonation. There is no significant back-donation noticed in 13 because of the highly electronegative Cl atoms bonded with the Ni center. The electronic scenario proposed using NLMO analysis were further studied by the CDA (charge decomposition analysis)

15

N

almost the same. Because the ionization potential of Si is slightly higher than that of Ni, the Si analogue is expected to exhibit greater covalent character. Earlier theoretical calculations10c,12a,d show that the π-backdonation from Si to CcAAC is greater in (cAAC)2Si (11) than that of Ni in (cAAC)2Ni (14). This indicates greater covalent character for the C−Si bond. However, it is not reflected in the charge data (0.55 e for Si and 0.34 e for Ni). The extent of CcAAC←M0 π-backdonation is similar in the palladium (15) (−175.0 ppm) and platinum (16 and 17) (−180.0 ppm) analogues.21 However, the 15N chemical shift of (cAAC)2Ni (14) is not obtained because of the broadening of 1H NMR resonances. The π-backdonation is comparatively higher in the zinc analogue [singlet diradicaloid (cAAC)2Zn (18)10a complex] [δ (15N) −245.0] than those of the corresponding Ni/Pd/Pt complexes (Figure 4). The chemical shift values of 12 and 21 are the most upfield-shifted [below −300 ppm (Figures 3 and 4)], when compared to those of the known compounds. The 15N NMR chemical shift values of some of the representative NHC-containing compounds have been recorded (Figure 5). The chemical shift in the 15N NMR

Figure 5. Representative diamagnetic NHC compounds with chemical shift values (δ).

15

N

spectrum of the free iPrNHC (Figure 5)14d appears at −185.1 ppm, which is much more upfield-shifted than that of cAAC (−159.0 ppm). The corresponding chemical shift value of the protonated carbene salt (iPrNHC·HCl) is further upfield-shifted (−196.3 ppm). This trend of iPrNHC/iPrNHC·HCl is the opposite of those of cAAC/cAAC·HCl (Figure 3), which might be due to the polarization of electron density of the CC bond toward the N atoms. Importantly, a similar CC bond is absent within the cAAC molecule. The chemical shift values of (iPrNHC)SiCl222 and (iPrNHC)2NiCl223 are −191.8 and −190.9 ppm, respectively, which are slightly upfield-shifted compared 166

DOI: 10.1021/acs.inorgchem.5b02055 Inorg. Chem. 2016, 55, 163−169

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Inorganic Chemistry Table 1. 15N Chemical Shift Values (recorded via 1,3-coupling of 1H with 15N)a

no.

CcAAC−N bond distance (Å)

Cy-cAAC·LiOTf

1

1.315(3) (1.303)

−159.0

(Cy-cAAC)Si(Cl2)P-Tip Cy-cAACH+OTf−

2 3

1.301 (1.300) 1.290 (1.287)

−160.7 −148.1

(Me2-cAAC)SiCl4

4

1.303(2) (1.303)

−164.1

(Cy-cAAC•)(Cl2)Si−Si(Cl2) (•CAAc-Cy) (Me2-cAAC)(Cl)Si−Si(Cl) (CAAc-Me2) (Me2-cAAC)SiSi(CAAcMe2) [(Me2-cAAC)PPh2]+Cl−

5

1.376(6) (1.379)

no signal

6

−208.5

7

1.336(3)−1.337(2) (1.339) 1.342(5) (1.343)

8

1.302(18) (1.300)

−130.3

(Me2-cAAC)O

9

1.374(10) (1.365)

−241.0

(Cy-cAAC•)2SiCl2

10

no signal

(Cy-cAAC)2Si

11

(Cy-cAACH)Si(CMe2) CAAc-Cy

12

1.400(2)−1.403(2) (1.392) 1.382(16)−1.373(16) (1.368) 1.461(3) (1.465)

(Me2-cAAC)2NiCl2

13

(Me2-cAAC)2Ni

14

−256.5 −160.6, −161.0 no signal

CH2 and CMe2 −

(Cy-cAAC)2Pd (Me2-cAAC)2Pt (Et2-cAAC)2Pt (Me2-cAAC)2Zn (Me2-cAAC)Se (Me2-cAAC)2Ge (Me2-cAACH)2O

15 16 17 18 19 20 21

−175.0 −180.1 −180.0 −245.0 −187.5 −222.5 −307.9

(Me2-cAAC)2Si2S4

22

1.383(3) (1.374) 1.3154(18)− 1.3148(18) (1.307) 1.3381(16)/1.3420(16) (1.332) 1.3267(18) (1.314) 1.3231(15) (1.324) 1.3267(18) (1.325) 1.376(2) 1.323(2) (1.337) 1.3666(19) (1.378) 1.4352(18)− 1.4353(19) 1.308(5) (1.306)

−173.0

compound

% of σdonation

% of πbackdonation

−150.1

6

−

23

−152.9 −167.3

23 35

− −

28 27

−172.2

24

−210 for E = Si; 2, 4, 6, and 7) of the 15N nuclei infer that CcAAC→E σ-donation is higher than CcAAC←E π-backdonation. When the numerical value of δ is smaller than −220 (for E = Si), the CcAAC←Si πbackdonation is stronger than CcAAC→Si σ-donation, indicating a higher extent of donor−acceptor partial double bond (CcAACSi) such as in silylone (11) and germylone (20) (Table 1). When the carbene bears a covalently bound hydrogen atom such as in 12 and 21, the 15N NMR values appear below −300 ppm. The theoretical NMR calculations strongly resemble the experimental values. The calculated data and the bonding analysis provide convincing evidence of the electron donation and backdonation from CcAAC and E/M, respectively. These promising results might encourage the researchers working with cAAC carbenes to study their compounds by 15N HMBC NMR spectroscopy and include additional 15N HMBC NMR chemical shift values (δ) along with other characterization data to provide an estimation of σdonation (CcAAC→E/M) and π-backdonation (CcAAC←E/M).

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